Method for removing and inserting optical carriers in a WDM optical communication system

ABSTRACT

An optical cable television system provides a large channel capacity and uses shared resources, whereby the more expensive hardware is shared over a large number of customers so as to minimize the cost per customer. A video switch network is provided which minimizes the temptation of subscribers to pirate signals since pay per view or on demand signals are provided only upon request of someone within a node corresponding to a neighborhood. An on demand program center continuously plays programs, such as movies, at time-staggered starts such that any subscriber will have only very minimal delays before seeing a particular program which is requested. A tuneable optical filter is provided in order to switch video signals onto an optical fiber going to the node in a particular neighborhood. An arrangement uses in-fiber grating in order to remove and insert different optical frequencies. Local insertion of various program sources, such as local commercials, is accomplished effectively by making such insertion at a relatively high point in the video distribution chain. An interface arrangement to provide on demand video signals to telephone company twisted pair wires is utilized.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. patent applicationSer. No. 08/071,263, filed Jun. 4, 1993, the disclosure of which isincorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to an optical cable TV system.

Various cable TV systems, often called CATV, have been used for deliveryof video signals to customers. Such cable TV systems have generally usedcoax cable to carry the video signals. Generally, such cables arelimited to carrying 100 or fewer channels or video signals.

In an effort to increase the channel capacity of cable TV systems,various proposals have been made to send the video signals along opticalfiber cables. Although such designs provide increased channel capacitybeyond that of coax cable, channel capacity is still more limited thanis desirable.

Apart from limitations on the channel capacity of cable TV systems,various other problems and constraints are generally present in cable TVsystems.

Cable TV systems often use a relatively large number of amplifiers whichare staggered along a trunk line in order to provide cable TV service.For example, there may be as many as seven amplifiers along a trunkline. Since each amplifier introduces at least some distortion, thequality of the video signal for those customers at the output of theseventh amplifier is much lower than the quality of the video signal forthose customers closer to the beginning of the chain of amplifiers.Additionally, the failure of one of the amplifiers near the beginning ofthe chain will cause a loss of service for all customers further downthe chain. If the first or second amplifier in the trunk line goes outof service, a very large number of customers will lose their cable TVservice until repairs can be made.

Various designs have been used to provide on demand video services,often called pay per view. Although such services have been useful inallowing customers to customize the shows which they want to watch,present on demand features have certain disadvantages. Generally, suchon demand or pay per view features provide the customer with a scrambledsignal and the customer can watch the on demand or pay per view signalonly if the customer signals the cable system that he or she wishes towatch the on demand signal. The cable TV system then usually sends datato the cable TV box at the viewer's house such that the cable TV box nowunscrambles the signal. Since the signal is generally sent to theviewer's house whether the viewer has paid for it or not, there is asignificant number of viewers who will buy descramblers or otherwisemake efforts to view the program without paying for it. Such viewing ofpirated pay per view or on demand video signals is a serious problem inthe cable TV industry. Further, such an arrangement requires adescrambler, controllable by the cable TV company, within the cable TVbox of all viewers. This increases the cost of equipment for the cableTV company, especially considering that a relatively sophisticateddescrambler is needed at the customer's cable TV box to try to minimizethe risk that the customer will buy or build their own descrambler inorder to pirate the pay per view signals.

Generally, the need for various components, such as descramblers,associated with each cable TV box at the customer's home, increases thecost of hardware which must be provided by the cable TV company. On theother hand, it has not been practical generally to move some of the moresophisticated switching or descrambling components out of the customer'scable TV box since moving those components to a more central locationwould usually interfere with the ability of the customer to customselect features which he or she wishes to view.

Although various systems have been developed on cable TV to providemovies in response to customer's selections, such on demand or pay perview programming has been quite limited in flexibility. For example, ifthe viewer wishes to watch a pay per view movie when sitting down fortelevision at 8:40 at night, the viewer may be discouraged to learn thatthe pay per view movie which he or she was interested in started at8:30. The customer then must start the movie 10 minutes late, wait untilthe next showing at 10:30, or simply forget about watching the pay perview movie that night. Since such movies or other pay per view videosignals are shown only at a relatively limited number of times, theviewer must accommodate the cable TV system instead of the other wayaround.

A problem with various optical cable TV systems is a difficulty inproviding adequate filtering which will filter out undesired opticalsignals with a high degree of rejection, while passing a desired opticalsignal. If one is to use an optical fiber for carrying video signals ondifferent wavelengths of laser light, one must have a tuneable opticalfilter (difficult to achieve satisfactorily) in order to select thewavelength of laser light corresponding to the desired signal or onemust have a plurality of dedicated optical filters (each optical filterdedicated to a single wavelength) and an arrangement for selecting fromthe outputs of the different dedicated optical filters. In either case,complexity, high cost, and other difficulties have generally beenencountered.

Cable TV systems often provide arrangements whereby one can blocktransmission of a signal such that a local signal can be inserted. Forexample, if a nationwide cable channel is provided to various localcable TV systems, such local cable TV systems want to be able to insertlocal commercials. At designated times in the feed from the nationwidecable channel, blocks of time will be provided for the local cable TVcompany to insert a local commercial. Arrangements for removal andinsertion of signals upon a channel are often quite complex andexpensive. Further, such local insertions often must be made at severallocations in order to cover a metropolitan area. This increases thehardware requirements for making such insertions and renders the processmore complex.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to providea new and improved optical cable TV system.

A more specific object of the present invention is to provide animproved optical cable TV system having substantially increased channelcapacity.

Yet another object of the present invention is to provide an opticalcable TV system wherein several of the more expensive portions of thesystem may be allocated over a relatively large number of viewers so asto minimize the cost per viewer of such expensive hardware components.

Yet another object of the present invention is to provide optical cableTV with an improved resistance to pirating of signals.

A still further object of the present invention is to provide an opticalcable TV system which provides substantially improved flexibility indelivering pay per view or on demand video signals.

A still further object of the present invention is to provide animproved tuneable optical filter.

Yet another object of the present invention is to provide for removaland insertion of local commercials or similar local features in a highlyefficient and relatively low cost fashion.

A still further object of the present invention is to provide an opticalcable TV system which reduces the risk that a single component failurewill cause loss of cable services for a large number of customers.

A still further object of the present invention is to provide opticalcable TV where customers have very clear signals regardless of wherethey are located in a distribution chain.

A still further object of the present invention is to provide an opticalcable TV system with minimal interference from one channel to another.

More generally, the present invention is designed to minimize or avoidthe problems discussed in the background portion of this application.

The above and other objects of the present invention which will becomemore apparent as the description proceeds are realized by a cable TVsystem including an on demand optical fiber bus carrying a plurality ofoptical wavelengths with on demand video signals. A plurality of videoswitch networks are used, each connected to the on demand optical fiberbus and including a plurality of parallel signal paths therein. Eachsignal path has a selector therein for selecting one of the on demandvideo signals. A plurality of network output optical fibers areprovided, each operatively connected to a corresponding one of the videoswitch networks to carry those on demand video signals selected by theselectors within that corresponding one of the video switch networks.The network output optical fibers extend from a central location remotefrom subscribers downstream towards the subscribers and carrying the ondemand video signals downstream only upon selection by the selectors.

The present invention may alternately be described as a cable TV systemhaving an on demand program center including a plurality of storagemeans having various video programs stored therein. Means are providedto stagger the start of the video programs such that each video programis started once every time interval T, which time interval is notnecessarily the same for each of the video programs. At least one laseris provided in the program center and has a modulator for modulatinglight from the laser based on one or more of the video programs. An ondemand optical fiber bus extending outside of the program centerreceives the modulated light and carries it to a first downstreamdistribution level. A video switch network at the first downstreamdistribution level is connected to the on demand optical fiber bus. Anetwork output optical fiber is operably connected to the video switchnetwork and extends to a second downstream distribution level, furtherdownstream than the first distribution level and remote fromsubscribers. The video switch network applies signals corresponding tothe video programs only upon subscriber request.

The present invention may further be described as a tuneable opticalfilter including a directional optical transfer device. A first opticalfiber carries a plurality of optical carriers to the transfer device. Asecond optical fiber is connected to the transfer device for receivingthe plurality of optical carriers. A plurality of tuneable in-fibergratings are disposed in the second optical fiber. Controllers fortuning each of the gratings allow reflection of a desired opticalcarrier back to the transfer device. A third optical fiber is connectedto the transfer device for receiving optical carriers reflected back tothe transfer device by the gratings.

The present invention may alternately be described as a remove andinsert system for removing and inserting optical carriers including afirst directional optical transfer device. A first optical fiber carriesa plurality of optical carriers to the first transfer device. A secondoptical fiber is connected to receive the plurality of optical carriers.A plurality of tuneable in-fiber gratings are in the second opticalfiber. Controllers are provided for tuning each of the gratings toreflect desired optical carriers back to the first transfer device. Athird optical fiber is connected to the transfer device for receivingoptical carriers reflected back. A fourth optical fiber is connected byan isolator to a second directional optical transfer device, theisolator blocking signals from passing from the first transfer device tothe second transfer device. A fifth optical fiber is connected to thesecond transfer device and has optical carriers corresponding to one ormore optical carriers not supplied to the third optical fiber due tonon-reflection from the gratings of the first transfer device. A sixthoptical fiber is connected to the second transfer device and has aplurality of in-fiber gratings therein for reflecting optical carriersto be added to the third optical fiber. Controllers are provided forcontrolling each of the gratings in the sixth optical fiber.

The present invention may alternately be described as an optical cableTV system including a metropolitan hub and a plurality of metropolitancells downstream from the hub and connected by optical fiber thereto. Aplurality of head ends are downstream from each of the cells and areconnected by optical fiber thereto. Insertion networks insert localprogramming are provided at a higher distribution point than necessaryfor a distribution zone for the local programming which is to beinserted.

The present invention may alternately be described as a system forapplying on demand video signals to a telephone twisted pair fordistribution. A tuneable optical filter is used for selecting an opticalcarrier having a desired program. Means receive the optical carrier andprovide an electrical output based thereon. A demodulation systemdemodulates the electrical output. A telephone modulator modulates thedemodulated output in telephone format. A switch network selectivelyapplies signals from the telephone modulator to one or more of aplurality of twisted pairs connected to the switch network.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will be morereadily understood when the following detailed description is consideredin conjunction with the accompanying drawings wherein like charactersrepresent like parts throughout the several views and in which:

FIG. 1 shows an optical cable TV system according to the presentinvention;

FIG. 2 shows more details of a regional transmitter portion of thesystem of FIG. 1;

FIG. 3 shows a portion of an alternate system from that of FIG. 1;

FIG. 4 shows a video switch network according to the present invention;

FIG. 5 shows a combiner which may be used in place of a coupler in thearrangement of FIG. 4;

FIG. 6 shows various optical wavelengths and is used for explaining theoperation of FIG. 5;

FIG. 7 shows a modification of a selector portion of the arrangement ofFIG. 4;

FIG. 8 shows a further alternate modification for the selector portionof FIG. 4;

FIG. 9 shows a first embodiment of a tuneable optical filter accordingto the present invention;

FIG. 10 shows a second embodiment tuneable optical filter according tothe present invention;

FIG. 11 shows a feedback control arrangement which may be used with thetuneable optical filters of the present invention;

FIG. 12 shows a third embodiment tuneable optical filter according tothe present invention;

FIG. 13 shows a remove/insert arrangement according to the presentinvention;

FIG. 14 is a wavelength diagram illustrating some of the principles ofoperation of the arrangement of FIG. 13;

FIG. 15 shows a simplified view of a portion of the structure of thedistribution network;

FIG. 16 shows an arrangement for insertion of different video signalsthan those from a primary source;

FIG. 17 shows an insertion network and related components which may beused to insert local programming;

FIG. 18 shows various radio frequencies indicating the allocation ofthose frequencies for a particular aspect of the present invention;

FIG. 19 shows portions of a modified video switch network providingvarious computer functions to a subscriber;

FIG. 20 shows details of an on demand program center according to thepresent invention;

FIG. 21 is a time chart illustrating certain principles of operation ofFIG. 20;

FIG. 22 is a more detailed time chart indicating a particular aspect ofoperation of the arrangement of FIG. 20;

FIG. 23 is a modification of a portion of the program center of FIG. 20;

FIG. 24 is a video selection arrangement for use in providing on demandvideo signals by way of telephone company twisted pairs of wires;

FIG. 25 shows an optical amplifier arrangement;

FIG. 26 shows a gain curve to illustrate operation of the FIG. 25amplifier;

FIG. 27 shows an optical source head end configuration;

FIG. 28 shows a daisy chain modification of the FIG. 27 arrangement;

DETAILED DESCRIPTION

Turning now to FIG. 1, a simplified block diagram shows an optical cableTV system 10 according to the present invention. A regional transmitter12 uses lasers 14, only three of which are shown for simplicity, andoptical coupler 16 to provide various video signals or channels onoutput optical fiber 12U. The coupler 16 simply uses known technology tocombine the N different wavelengths corresponding to the N differentlasers 14, the laser energy being supplied to coupler 16 on opticalfibers 14F. As will be discussed in more detail below, each of the videoinputs indicated for the lasers 14 may carry signals corresponding to arelatively large number, such as 3,000 different video signals orchannels. The output from the regional transmitter or hub 12 is fedalong optical fiber 12U to pass sequentially through distributionoptical amplifiers 18 having various optical fibers extendingtherebetween and having optical signal splitters (not separately shown)separating out from the optical fibers between the various amplifiers 18in order to go to the various metropolitan hubs 20 corresponding todifferent metropolitan areas. The optical signals received on theoptical fiber by each metropolitan hub is sent, still in optical form,along various optical fibers 22 to optical amplifiers 24 and on to headends 26. For ease of illustration, only some of the optical fibers 22,amplifiers 24, and head ends 26 are labeled. The head end 26 splits theoptical signals for feeding along a plurality of optical fibers 26F to aplurality of neighborhood nodes 28. For ease of illustration, only twoof the optical fibers 26F are shown and only two nodes 28 are shown.However, it will be understood that significantly more than two of theoptical fibers 26F extend to corresponding significant numbers of nodes28 from each of the head ends 26. Each of the head ends 26 would have aplurality of the nodes 28 connected to it, each node corresponding to aparticular neighborhood with, for example, between 500 and 2,000subscribers. The node 28 converts the optical signals on optical fiber26F to electrical form for coax distribution. As shown just to the rightof the right-most node 28 in FIG. 1, the resulting radio frequencysignals are fed along coax 28C to RF amplifiers 30 to a subscriber'shome 32. Although the signal is shown passing through two RF amplifiers30 before reaching the subscriber's house 32, other houses (not shown)connected to the same node 28 may have signals which pass to the houseswithout passing through the two amplifiers. Advantageously, the use ofseparate optical fibers 26F connecting each node 28 to the correspondinghead end 26 avoids the need for a plurality of distribution amplifiersin that portion of the path. Thus, none of the nodes 28 are further outon the distribution chain from head end 26 than any of the other nodes28 and customers at any of the nodes 28 should have a picture ofsubstantially the same clarity.

An on demand program center 34 may also be used to supply on demand orpay per view type programming to the various nodes 28. The on demandprogram center 34 includes a video server 36, video on demand block 38,and video dial tone platform 40, all of which may work in known fashionusing known components except that they may incorporate certain highlyadvantageous features which will be discussed in more detail below.

Video signals from the video dial tone platform 40 are supplied tolasers 42, only two of which are shown. The outputs of the lasers 42 arefed to a coupler 44 having various output optical fibers 44F, only twoof which are shown. Each of the output optical fibers 44F carries all ofthe signals entering the summer 44 through the different optical fibersfrom the lasers 42. Each of the optical fibers 44F proceeds towards acorresponding node 28 by way of several intermediate components. Inparticular, each of the optical fibers 44F goes to a video switchnetwork 46. The details of the video switch network 46 will be discussedbelow. Each of the optical fibers 44F carry all of a very large numberof signals. The video switch network selects a significantly smallernumber of the signals to proceed by coax to modulate laser 48. The videoswitch network 46 has converted the video signals from their opticalform on optical fibers 44F such that coax is used to provide the samesignals to lasers 48. The lasers 48 then supply the selected videosignals, in optical form, to node 28. Although the video switch networks46 and lasers 48 might be located at the node 28, it is more preferableto have them located at the same physical location as the head end 26.Although the arrangement of FIG. 1 shows the optical fibers 44Fproceeding directly from summer 44 to the video switch networks 46, onecould alternately feed the output of summer 44 through a distributionchain similar to the distribution chain of amplifiers 18 and hubs 20shown with respect to the output of summer 16. Additionally, if desired,the outputs of lasers 42 could be supplied to the summer or signalcombiner 16 for distribution along optical fiber 12U and thedistribution chain downstream thereof. In that case, the various headends 26 might have some arrangement to separate out the on demandsignals so that only the signals requested by subscribers within aparticular neighborhood node 28 would be sent to that neighborhood node28.

Regardless of the various possibilities for distributing the videosignals from program center 34 discussed above, a key feature is thatthe on demand or pay per view programming is not supplied to thesubscriber's house 32 unless someone in his neighborhood has requestedthe particular on demand or pay per view video signal. In other words,the video switch network 46 only supplies programs to a particular node28 if someone within that node has indicated a request for theparticular on demand or paid per view program. Since other subscriberswithin a particular neighborhood corresponding to node 28 will not becertain which program is on which frequency and which program has beenordered, the motivation for piracy will be greatly reduced. A subscriberin a particular house 32 is significantly less likely to try to pirate avideo signal if he simply does not know what will be on the signal.Therefore, relatively well known and inexpensive blocking arrangements(not shown) can be used to block the pay per view or on demandprogramming from reaching customers within the neighborhoodcorresponding to node 28 when their neighbor within that node hasselected a particular program. A video selection computer 50 is used tocontrol the video switch network 46 and is connected by coax to thesubscriber's home 32 and the video switch network 46. Preferably, therewould be one video selection computer 50 for each of the video switchnetworks 46 (although only one selection computer 50 is shown) and theselection computer 50 would be connected to all subscriber's homes in aparticular neighborhood corresponding to the node 28. When thesubscriber in home 32 wishes to view a particular program, the videoselection computer is signalled by the subscriber using a key pad orother input means (not shown). The selection computer 50 then causes thecorresponding video switch network 46 to pass the appropriate videosignals through to the corresponding laser 48.

Turning now to FIG. 2, more of the details of the regional transmitter12 will be shown including components left out of FIG. 1 for ease ofillustration. As shown in FIG. 2, a relatively large number of lasernetworks 52 provide optical signals on optical fibers 14F to the coupler16. The laser networks 52 provide video signals on optical fibers 14F atdifferent wavelengths, each laser network 52 corresponding on aone-to-one basis with a particular optical wavelength. Each of the lasernetworks 52 may be identically constructed in a fashion which will beillustrated with respect to the first laser network. There may be asmany as 128 laser networks 52 as illustrated. Each of the laser networks52 may provide 3,000 video channels or signals on its correspondingoptical fiber 14F. However, if desired, some of the laser networks 52may provide various supervisory data (not separately shown) on thecorresponding optical fibers 14F.

Turning to the details of the first laser network 52, network 52includes 300 time division multiplexors 54, each of which receives 10different video input signals such as V₁ through V₁₀ as illustrated withrespect to the left most time division multiplexor 54. Since 10different video signals are provided to each of the 300 multiplexors 54,300 channels or video signals are provided in total to the various timedivision multiplexors 54 within a particular laser network 52. The 10video signals, received in electrical form by the time divisionmultiplexors 54 are multiplexed and passed along, still in electricalform, by wire to corresponding quadrature amplitude modulationmodulators 56. Preferably, each of the modulators 56 uses 64 QAM. Theoutputs of the 300 different modulators 56 are 300 different radiofrequency carriers, each having 10 video signals impressed thereon.Those radio frequency carriers are supplied by wire from modulators 56to an adder 58 which combines the different inputs. The output of adder58 is supplied to an optical modulator 60, which may be aninterferometer. Accordingly, the 3,000 different video signals for theparticular laser network 52 are used to modulate the output of laser 14.The output of optical modulator 60 is supplied to an optical amplifier62 before proceeding to the coupler 16.

Different video signals would be supplied to each of the laser networks52. The same 300 different radio frequency carriers may be used by the300 modulators 56 within each of the laser networks 52 since those RFcarriers will be used to modulate different optical wavelengths in therespective different laser networks 52. Accordingly, it will beappreciated that a huge number of channels or video signals may besupplied on optical fiber 12U.

Next turning to FIG. 3, there is shown a simplified view of an alternateto the structure of FIG. 1. For ease of discussion, components in theFIG. 3 arrangement have the same last two digits as the correspondingcomponent, if any, in the arrangement of FIG. 1. In FIG. 3, the node 128corresponding to a particular neighborhood converts video signals inoptical form to electrical form for proceeding along coax cables toamplifiers 130 and houses 132 by way of taps 131. As illustrated, thenode 128 is connected by optical fiber 128F to a video phone signalprocessing network 164. The signal processing network 164 may providevideo phone and voice phone by optical fiber 164F. Additionally, a videoselection computer (not shown in FIG. 3) may be disposed withinprocessing block 164 and used to control the video switch network 146 asdiscussed previously with respect to the video switch network 46 ofFIG. 1. If a customer in a particular house 132 wants to view aparticular on demand or pay per view feature, the subscriber uses atelephone key pad, dedicated keyboard, or other control (not shown) tosupply a request by way of node 128 to signal processing block 164 whichin turn causes the video switch 146 to pass selected video signals froma video on demand optical fiber bus 146B (containing one or more opticalfibers) to modulate a laser 148 (separate modulator not shown) by way ofa coax cable 146C. The laser 148 supplies the selected video signals, inoptical form, to an optical fiber 148F which proceeds to the node 128.Since the video switch 146 and laser 148 are located remotely from thesubscriber's home 132, there is significantly less incentive for asubscriber to try to pirate signals because the subscriber will beunaware of which signals would be supplied at a particular time. If thesubscriber's neighbor selects a pay per view sporting event, it would besupplied to the subscriber's house 132. However, the subscriber wouldnot know what channel it was on, and would not normally know if theneighbor had even ordered the sporting event. He would therefore besignificantly less inclined to try to pirate the signal. Under thosecircumstances, very simple known arrangements (not shown) could be usedto insure that only the person paying for the sporting event or other ondemand or pay per view video signal was able to decode that signal. Inother words, the large number of channels and uncertainty over whatsignals were coming into the subscriber's house means that any blockingarrangement within the subscriber's house can be relatively inexpensivewithout fear of major piracy problems.

The arrangement of FIG. 3 is different than FIG. 1 also in that head end126 may receive various broadcast signals by satellite dish 126D. Thebroadcast signals are supplied by wire to a laser 114. Although notshown, the video signals from head end 126 may be impressed upon thelaser energy from laser 114 using the same techniques as shown withrespect to the first laser network 52 of FIG. 2. The output of laser 114is fed to an optical amplifier 162 for passage to a coupler 116. Thecoupler 116 is different than coupler 16 of FIG. 1 in that coupler 116has a plurality of optical fibers 116F extending therefrom. Each of theoptical fibers 116F would go to a different node 128, although only onenode 128 is shown in the drawing for simplicity. Likewise, the othernodes 128 which are not shown would have separate video switches 146,lasers 148 and video phone signal processing networks 164 correspondingto them.

With reference now to FIG. 4, the details of the video switch network 46of FIG. 1 will be explained, it being understood that video switchnetwork 146 of FIG. 3 is identically constructed. The various videosignals, in optical form, on optical fiber 44F are provided to a coupler200 which provides optical outputs on optical fibers 200F, all of whichare identical to the optical signals on the input optical fiber 44F.Referring back momentarily to FIG. 1, and considering that the videosignals may be impressed on the laser energy from lasers 42 in the samefashion as shown with respect to the first laser network 52 of FIG. 2,there may be over 100 different optical wavelengths on optical fiber 44Fand all of those optical wavelengths would be supplied to the opticalfibers 200F. A selector 202 receives the over 100 optical wavelengths.Each of the optical wavelengths on optical fiber 200F may carry 3,000channels or video signals in the fashion described with respect to firstlaser network 52 of FIG. 2. Each of the selectors 202 is used to selectone of the video signals or channels from among the over 300,000channels (it might be more or less) provided on optical fiber 200F. Forease of illustration, the details of selector 202 are given for only oneof the selectors 202. Additionally, only one other of the selectors 202is shown. However, in practice a relatively large number of selectors202 would be used. For example, if experience shows that a neighborhoodmight have subscribers asking for 64 pay per view or on demand videosignals or channels at a particular time, there would be 64 of theselectors 202, each receiving identical optical inputs on the opticalfibers 200F.

The selectors 202 essentially partially reverse the process discussedwith respect to the first laser network 52 and coupler 16 of FIG. 2.Initially, a tuneable optical filter 204 selects a single one of thewavelengths corresponding to the channel or video signal which asubscriber in the neighborhood of the node corresponding to video switch46 wishes to view. As mentioned, there may well be over 100 of theoptical wavelengths supplied by each of the optical fibers 200F. If asubscriber in the neighborhood selects a channel or video signal whichis carried by the third wavelength, the video selection computer 50(refer back momentarily to FIG. 1) supplies a signal on control line204C such that optical filter 204 will allow passage of the thirdwavelength through to optical fiber 204F. The optical filter 204, whichis a band pass optical filter, will be described in more detail below.At this stage, it is sufficient to note that the optical filter 204rejects all of the wavelengths of laser energy except the particularone, such as the third wavelength, which is carrying the channel orvideo signal requested by the subscriber. The selected wavelength oflaser energy is supplied to photodiode 206 from which the up to 3,000channels or video signals are fed in electrical form to an amplifier208. The output of amplifier 208 is fed to a multiplier 210 formultiplication with a radio frequency signal from local oscillator 212.The tuneable local oscillator 212 provides a radio frequencycorresponding to the radio frequency which is carrying the desiredchannel or video signal. Specifically, a control 212C is used to controlthe local oscillator 212, using known technology, such that the outputof multiplier 210 corresponds to the 10 channels on the radio frequencyselected by local oscillator 212. The control 212C setting thatfrequency would be connected to the video selection computer 50 (FIG. 1only). The output of multiplier 210 is supplied to an intermediatefrequency filter and from there on to amplifier 216. The output ofamplifier 216 is supplied to a multiplier 218 which also receives asignal from local oscillator 220 as set by control 220C (from the videoselection computer 50 of FIG. 1) and are part of a superhetrodynedetection arrangement. Although the arrangement could demultiplex toselect one out of the 10 time division multiplexed signals supplied,such TDM demultiplexing would preferably be done at the subscriber's settop cable box. The output of multiplier 218 is supplied to an adder 222.Each of the selectors 202 is part of a corresponding parallel signalpath going from coupler or splitter 200 to the adder 222. Each of theparallel signal paths selects one of the channels for passage to theadder 222. If desired, more than one of the optical fibers 44F (notshown) could be used to supply different couplers 200 (only one shown).In that case the optical fibers 44F might carry different signals andsupply different selectors 202 by way of different couplers or signalsplitters 200. The different optical fibers 44F could be part of a videoon demand bus such as bus 146B of FIG. 3. In the case where a pluralityof optical fibers 44F carry different signals, the selectors 202 wouldbe limited to selecting from among the signals supplied to them by thecorresponding coupler 200. However, the outputs from all of theselectors 202 could be supplied to the same adder 222. The output of theadder 222 may be used to modulate the output of laser 48 on opticalfiber 48F. This is shown in simplified form as various modulationarrangements, such as those shown and discussed with respect to FIG. 2,could be used. If desired, simpler modulation techniques could be useddepending upon the number of channels or video signals which will besupplied out of the adder 222.

Again referring to the example where there would be 64 of the selectors202 for a particular video switch 46, each of the selectors 202 wouldoutput a particular video signal or channel requested by someone withinthe neighborhood corresponding to that video switch. By modulating 64different radio frequencies with the 64 video inputs, the resulting 64modulated radio frequency signals could be combined and used to modulatethe light from laser 48.

By having the video switch network 146 of FIG. 3 with its associatedoptical and radio frequency tuning arrangements at the head end locationor central switching location in the cable TV system, significantequipment cost is reduced. This arrangement allows resource sharing inthat an individual subscriber may use one of the channel selectorstructures 202 of FIG. 4 to pull in or receive any program from thevideo on demand bus 146B of FIG. 3. The individual subscriber can havethat program placed upon optical fiber 148F and delivered to the node128 in his neighborhood. Since the optical and radio frequency tuningand the time switching functions are maintained at the head end, theseresources are time-shared by a large number of consumers or subscribers.An example using specific numbers may be useful in explaining this. If anode corresponds to 500 different subscribers or households, one wouldrequire 500 different units like selector 202 in order to provide allthe subscribers with the access to such a large number of channels asthe present system provides. Assuming that the system anticipated a peakusage of about 10% for the on demand channels, the present arrangementwould allow 50 of the selectors 202 to be located at the head end orcentral switching office so that 10% of the 500 subscribers could bewatching on demand signals at any one time. Therefore, the costassociated with the hardware of selector 202 is reduced to one tenth ofwhat it would otherwise be by virtue of its location at the head end orcentral switching office. In addition to the substantial savings in thecapital cost of the selectors 202, installation and maintenance aresignificantly less expensive when the equipment, especially the moresophisticated equipment, is installed at the head end or centralswitching location.

The node 128 of FIG. 2 may simply be a photodiode which receives opticalenergy from optical fibers 116F and 148F and supplies the output to aradio frequency amplifier. In that case, it is important to note thatthe laser 114 and laser 148 should operate at different opticalwavelengths. Additionally, the radio frequency subcarriers that are usedby laser 114 and laser 148 should be mutually exclusive to preventinterference once the signals have been changed from the optical domaininto the radio domain.

With respect to the video phone signal processing box 164 and the videophone bus 164F of FIG. 3, it should be noted that the video phone buswould carry signals in both directions such that an amplifier chain (notshown) would be arranged running in both directions. Further, it shouldbe understood that the video phone bus may be carrying data such aspersonal computer data from a subscriber and may be used for modempersonal computer communications. Other data and information could ofcourse be transmitted in either direction.

With reference now to FIGS. 5 and 6, a minor variation for parts of thearrangement of FIG. 4 will be discussed. The signals coming in onoptical fiber 44F of FIG. 4 may include 128 different wavelengths.However, for the discussion which follows, it will be assumed that only100 different wavelengths are used. If 100 wavelengths are used on asingle optical fiber such as 44F of FIG. 4, certain distortions mayoccur in the signal quality depending upon the closeness of thewavelengths and the length of the optical fiber. The arrangement of FIG.5 uses a plurality of input optical fibers 224N to supply an opticalsignal combiner 224 having output optical fibers 224F. The outputoptical fibers 224F will carry all of the wavelengths on each opticalfiber and the combiner 224 would be substituted in place of the coupler200 of FIG. 4. Accordingly, the output optical fibers 224F may lead toselectors 202 similar to that shown in FIG. 4. The important differencebetween combiner 224 and coupler 200 is that combiner 224 includes aplurality of input optical fibers 224N each of which only carries aportion of the total wavelengths. In particular, there would be 10 inputoptical fibers 224N, each of which would carry only 10 of the 100wavelengths which would be used.

With reference to FIG. 6, the 100 wavelengths which are used could bethought of as 10 wavelengths in each of 10 different wavebands, two ofthe wavebands being illustrated in FIG. 6. Wave band one includeswavelength W₁₋₁, wavelength W₁₋₂, through wavelength W₁₋₁₀. It will berecognized that the notation indicates the waveband by the first digitin the subscript and the number of the wavelength within a particularwaveband by the second digit within the subscript. Accordingly, wavebandtwo includes wavelengths W₂₋₁ through W₂₋₁₀. With reference to the topinput fiber 224N in FIG. 5, it will be seen that the top input fibercarries each of the first wavelengths within the 10 different wavebands.Since wavelength W₁₋₁ is separated in wavelength significantly fromwavelength W₂₋₁, there is much less distortion by having suchwavelengths travel on the same optical fiber than there would be ifwavelengths W₁₋₁ and W₁₋₂ traveled along the same fiber. Accordingly,and as illustrated with respect to the lower most of the input opticalfibers 224N, each of the 10 input optical fibers 224N includes only asingle wavelength from each of the 10 wavebands. All of the firstwavelengths within the wavebands travel on the top optical fiber, all ofthe second wavelengths within the 10 optical bands travel on the secondinput fiber 224N, etc. Although all of the wavelengths travel on each ofthe optical fibers 224F at the output of the combiner 224, the length ofthe optical fibers 224F is relatively short such that minimal distortionwould be introduced by having immediately adjacent wavelengths travelingon the same optical fiber.

With reference momentarily back to FIG. 1, it should readily beappreciated that the technique illustrated and explained with respect toFIGS. 5 and 6 could be used anywhere in the system where a lengthyoptical fiber is used. For example, instead of a single optical fiber12U in FIG. 1, a plurality of optical fibers (not shown) could be usedhaving wavelength distribution similar to that explained with respect toinput optical fibers 224N of FIG. 5. Couplers could be used to combinethe optical wavelengths or carriers at places in the system whererelatively short optical fibers carrying all optical wavelengths areneeded.

In the arrangement of FIG. 4, the subscriber has a set top decompressionbox which can select the desired subcarrier frequency so as to recoverthe required video channel. Other subscribers on the same node selectthe subcarrier channels required for the particular video channel whichthey want to view. Assuming the simple arrangement of having the outputof adder 222 used to modulate the laser 48 as shown in FIG. 4, the node28 or node 128 (refer to FIG. 1 or FIG. 3) may include a simple opticalreceiver to convert the signal back to the electrical domain. No signalprocessing other than amplification would be required. After the nodehas converted the signal to the electrical domain using a standardoptical receiver, the signal is passed via an active or passive coaxdistribution plant to the home where the set top decompression boxselects the subcarrier frequency as discussed.

FIG. 7 shows a modification of a portion of the selector 202 of FIG. 4.In particular, the modification of FIG. 7 differs from FIG. 4 in thatdigital decompression at the subscriber's set top would not be requiredif the arrangement of FIG. 7 is used. The arrangement of FIG. 7 includesa quadrature amplitude modulation demodulator 226 which would receive asits input the output of a band pass or intermediate frequency filtersuch as filter 214 of FIG. 4. Generally, the components of FIG. 7 wouldreplace the components shown in FIG. 4 to the right of multiplier 210 upto the adder 222 of FIG. 4. The demodulator 226 supplies a data recoveryblock 228 having adder 230, clock recovery 232, frame recovery 234, andchannel recovery 236 connected as shown. The details of data recovery228 need not be discussed since such data recovery blocks are wellknown. The data from channel recovery 236 and strobe developed by theblock 228 are supplied to a digital to analog and digital decompressionsystem 238 which, using known techniques, provides video and audiosignals corresponding to the requested channel. The video and audio forthe requested channel are supplied to an amplitude modulator 240receiving a signal from local oscillator 242. The output 240U ofamplitude modulator 240 would be supplied to an adder (not shown) likethe adder 222 of FIG. 4. In similar fashion, outputs from various otherselectors constructed in the manner illustrated for FIG. 7 could beprovided to the same adder.

The details of the digital decompression system 238 need not bediscussed since such decompression systems are well known. What isimportant about the arrangement of FIG. 7 is that the digitaldecompression chip may be shared by a large number of subscribers. Inother words, only one of the digital decompression chips or systems 238is required for each of the selectors 202. Assuming that 64 selectors202 (refer back momentarily to FIG. 4) are used, only 64 digitaldecompression chips (system 238 is usually a chip) would be required forall of the subscribers at a particular node. Therefore, decompression ofthe digital data is much cheaper (a shared resource among numerouscustomers) than it would otherwise be.

As a further alternative to FIG. 4, a modification of FIG. 7 could beused for the selectors if digitally compressed video signals are sentvia base band digital. The advantage is that base band digitaltransmission would provide a more robust transmission format. Thedisadvantage is that base band digital transmission requires morebandwidth than QAM. The alternative would have the components startingwith data recover block 228 to output line 240U extend between amplifier208 and adder 222 of FIG. 4.

As a further modification of the arrangement of FIG. 7, data and strobesignals from various of the data recovery blocks 228 may be fed to atime domain multiplexor 250 in FIG. 8. Thus, data 1 and strobe 1 comefrom one data recovery block such as 228, whereas data j and strobe jwould come from a data recovery block in a different one of theselectors (refer back momentarily to selectors 202 of FIG. 4). Likewise,data and strobes from the other data recovery blocks (not illustrated)would be supplied to the multiplexor 250. The output of multiplexor 250is supplied to a quadrature amplitude modulator 252. The output ofmodulator 252 is supplied to multiplier 254 which also receives a signalfrom local oscillator 256. The output of multiplier 254 is supplied online 254L to an adder 258 where it is added to outputs of several suchoutput lines 254L from several such multipliers 254 each of which wouldbe connected to a different time domain multiplexor (not shown) by wayof a corresponding quadrature amplitude modulator (not shown). The adder258 serves the same function as adder 222 in the FIG. 4 arrangement. Theoutput of adder 258 is used to modulate a laser 260 (which functionslike laser 48 of FIG. 4). The laser 260 will provide quadratureamplitude modulation on the subcarriers.

With reference now to FIG. 9, an arrangement for realizing the tuneableoptical filter 204 of FIG. 4 will be discussed. In the discussion whichfollows, it will also be useful to refer to FIG. 6 and assume that 100different wavelengths, 10 wavelengths in each of 10 wavebands, are beingreceived on optical fiber 200F. A tuneable Mach-Zehnder filter 300provides coarse filtering. In particular, the filter 300 selects thewaveband in which the desired channel is at. For example, assuming thatthe channel which is to be selected by selector 202 of FIG. 4 is achannel or video signal within waveband two (FIG. 6), the filter 300 isadjusted to allow passage of wavelengths W₂₋₁, W₂₋₂, up through W₂₋₁₀.Those 10 wavelengths within waveband two are supplied on output fiber300F to an optical circulator 302. As indicated by the arrow within thecirculator 302, the 10 wavelengths supplied by optical fiber 300F to afirst port 302F of circulator 302 pass out of port 302S to an opticalfiber 304 having a series of in-fiber Bragg grating elements 306Athrough 306J. The in-fiber Bragg grating elements or components 306Athrough 306J are of the known type developed by Meltz and Morey. Suchgratings 306A through 306J can achieve optical bandwidths of four GHz orless. As known, these Bragg gratings can be tuned by varying theirtemperature. The temperature tuning coefficient is 11 to 13 pm/degreescentigrade. By selecting the three dB optical bandwidth of the gratingsto be four Ghz and by setting the center reflection wavelength of thegratings on 0.35 nm centers (i.e., 1532 nm, 1532.35 nm, 1532.70 nm . . .1567 nm) at a nominal temperature of 20° C., a 45 GHz channel bandwidthestablished by the tuneable Mach-Zehnder filter 300 is divided into 10channels. In particular, the temperature of the individual Bragggratings 306A through 306J can be varied by plus or minus 16° C. Thus,100 different wavelengths can be selected over 35 nm, which correspondsto the output bandwidth of the Erbium fiber amplifier used as an opticalamplifier at various places within the system.

Each of the gratings 306A through 306J has a corresponding resistiveheating element 308A through 308J. As with the gratings, only some ofthe resistive heating elements 308A through 308J are illustrated. Eachof the resistive heating elements is connected to a control interfacecircuit 310 which simply converts a control signal on line 310N (whichsignal would be supplied by video selection computer 50 of FIG. 1). Thecontrol interface circuit 310 simply controls the resistive heatingelements 308A through 308J corresponding to the optical wavelength whichis desired. Assume that it is desired to select wavelength W₂₋₂, thefilter 300 allows passage of all of the wavelengths within waveband twowhich proceed along optical fiber 304. The first grating 306A withinoptical 304 would have been used to select wavelengths within wavebandone. Since the desired wavelength is not in waveband one (refer back toFIG. 6) and since the signals within waveband one have been filtered outby filter 300, grating 306A need not be tuned for selecting wavelengthW₂₋₂. Alternately, control interface circuit 310 might control resistiveheating element 308A such that grating 306A, which corresponds towaveband one, is tuned to a wavelength such as W₁₋₁ which will minimizeany reflection from grating 306A for the small amount of energy inwaveband one which passes through filter 300. In any case, grating 306Bwill be controlled in order to select wavelength W₂₋₂ from waveband two.(In similar fashion, grating 306C would correspond to the thirdwaveband, grating 306J would correspond to the tenth waveband, withsimilar gratings located in between). By controlling the temperature ofgrating 306B through resistive heating element 308B, the grating 306B istuned to reflect wavelength W₂₋₂. The grating 306B operates in knownfashion to reflect the desired wavelength and allow passage ofwavelengths other than the desired wavelengths. Accordingly, wavelengthW₂₋₂ is reflected back from grating 306B toward the second port 302S ofcirculator 302. Circulator 302 then supplies the selected wavelengthW₂₋₂ at third port 302T of circulator 302 for passage along opticalfiber 204F where it can be processed further in the manner describedwith respect to FIG. 4 above. (The circulator is a directional transferdevice and a directional optical coupler might be substituted for it ifisolators were also used.)

Turning now to FIG. 10, an alternate arrangement for the tuneableoptical filter will be discussed. In the arrangement of FIG. 10,components are numbered in the "400" series with the same last twodigits as the corresponding component in the FIG. 9 embodiment.Circulator 402 operates the same as circulator 302 of FIG. 9 andin-fiber Bragg gratings 406A through 406J operate as with the gratings306A through 306J of FIG. 9 with an important difference to be discussedbelow. For ease of illustration, FIG. 10 does not include the resistiveheating elements and control interface circuit used to control thevarious gratings, but it will be readily appreciated that an arrangementlike that of FIG. 9 would be used.

The arrangement of FIG. 10 avoids the need for a coarse tuneable filtersuch as 300 of FIG. 9. Since there is no filter similar to filter 300 ofFIG. 9, the arrangement of FIG. 10 uses an alternate technique forselecting the waveband. With reference to FIG. 6, the arrangement ofFIG. 10 requires that the waveband separation as shown is larger thanthe wavelength separation. In other words, the distance betweenwavelength W₁₋₁₀ and wavelength W₂₋₁ is significantly greater than thedistance between, for example, wavelength W₁₋₁ and W₁₋₂. (For thetechnique of FIG. 9, the waveband separation might be equal to thewavelength separation.) For the arrangement of FIG. 10, the separationbetween the top wavelength in one waveband and the lowest wavelength inthe next waveband provides what will be called an idler gap. The idlergap is at least as wide as the bandwidths B (only one labeled in FIG. 6)of the wavelengths summed with twice the wavelength separation. Themanner in which the idler gap technique works is best illustrated by anexample. Assume that wavelength W₂₋₂ is the wavelength which is to beselected. If the grating 406A was placed anywhere within waveband one,energy corresponding to its placement would be reflected back fromgrating 406A and would be applied by circulator 402 to optical fiber204F. However, by tuning the grating 406A to the center of the idler gap(using the same tuning technique discussed with respect to resistiveheating element 308A of FIG. 9), all of the energy in waveband one willpass through grating 406A. Since there is no energy or signal at thewavelength corresponding to the idler gap, grating 406A will not reflectany wavelengths which are present. Grating 406B would be tuned to selectthe wavelength W₂₋₂ and would reflect that wavelength in the samefashion as discussed above. By providing an idler gap in between eachpair of adjacent wavebands, the various gratings 406A through 406J canprovide all of the optical tuning without requiring a filter such asfilter 300 of FIG. 9. If one was selecting an optical wavelength withinthe third waveband (not shown in FIG. 6), each of gratings 406A and 406Bwould be tuned to their idler gaps such that they would reflect nowavelengths. In similar fashion if a wavelength within the tenthwaveband corresponding to grating 406J was to be selected, each of thenine proceeding gratings would be tuned to an idler gap. Since each ofthe gratings must be able to tune to the idler gap, this may slightlyreduce the number of wavelengths which can be used. For example, itmight be that each of the wavebands would only accommodate ninewavelengths and the idler gap would effectively correspond to the tenthwavelength which had been dropped from the waveband. Since the presentsystem provides such a high channel capacity, the slight reduction inchannel capacity may be worthwhile to avoid the need for a filter likefilter 300 of FIG. 9.

Turning now to FIG. 11, a feedback control circuit which may be usedwith the tuneable optical filter 204 of the present invention will bediscussed. FIG. 11 shows optical filter 204, photodiode 206, andamplifier 208 from FIG. 4. Additionally, it shows how the output ofamplifier 208 may be used as part of a feedback control loop in order toinsure proper operation of the optical filter 204. The output ofamplifier 208 is supplied to a multiplier 500 and a 90° phase shift 502.For the feedback control arrangement of FIG. 11, each of the 100 or sooptical wavelengths (refer back momentarily to FIG. 6) may have onesubcarrier which can serve two functions, identification of the opticalcarrier and carrying of supervisory data for the optical carrier orwavelength. If desired, the subcarrier tone in either an analog ordigital signal processing scheme may be directly used to generate anerror signal for locking the tuneable optical filter 204 to the requiredoptical carrier. It is desirable that adjacent optical carriers orwavelengths have different radio frequency subcarrier frequencies sothat the feedback loop will not lock to the wrong optical carrier orwavelength. However, instead of having 100 different RF subcarriers usedfor the approximately 100 different optical carriers or wavelengths, 10different RF subcarriers might be utilized. In addition, each RFsubcarrier would be modulated by a scheme such as FSK, ASK, QPSK, orQAM. This low data rate channel would carry unambiguous channelidentification as well as additional housekeeping and supervisory data.Once the analog feedback loop has locked to the desired subcarrierfrequency, the digital channel identification is checked. If the correctchannel has been locked to, the search is over. If an incorrect channelhas been found, a signal processing circuit will search another group.Assuming use of 10 different RF subcarriers, each of the 10 could beused for a corresponding one of the 10 wavelengths within a particularwaveband (refer back momentarily to FIG. 6).

The output of amplifier 208 is supplied to the multiplier 500 and the90° phase shift 502. A tuneable local oscillator 504 is set by control504C (from video selection computer 50 in FIG. 1) to the frequencycorresponding to the RF subcarrier of the desired optical carrier orwavelength. The output of oscillator 504 is supplied to the multiplier500 and to multiplier 506 (which also receives the output from the phaseshift 502). The outputs of the multipliers 500 and 502 are supplied tomatched filter 508. The output of the matched filter 508 is supplied tothe signal processing circuit 510, which also receives supervisory datasupplied from adder 512, frame recovery 514, and clock recovery 516arranged as shown. The signal processing circuit or processor 510generates an error signal supplied to filter control 518 if the opticalfilter 204 is deviating from the desired optical carrier or wavelength.Additionally, the signal processing circuit 510 would indicate from thesupervisory data if the feedback loop had somehow locked onto the wrongoptical carrier.

With reference to FIG. 12, an alternate tuneable optical filter will bediscussed. An in-filter Bragg reflective grating is used as part of thealternate tuneable optical filter 520. In particular, the grating 522 isselected with a value for R of 0.70 and an optical bandwidth of 50 pm ata wavelength of 1550 nm. By stretching this grating 1%, the wavelengthcan be changed to 1558 nm. Thus, the grating 522 can yield 80 opticalchannels or wavelengths if the channel spacing is set at 100 pm. For thetotal filter tuning of eight nm corresponding to 8,000 pm, 80 channelsof 100 pm are provided. This will provide an information bandwidth foreach optical carrier of about 2 to 3 GHz. If more information bandwidthis required, a slightly wider optical bandwidth could be used and thetotal number of channels could be reduced.

The optical wavelengths or carriers are provided on optical fiber 200Fto a directional coupler 524. The optical wavelengths on optical fiber200F pass through coupler 524 and on to optical fiber 526 to grating522. As shown, clamps 528A and 528B are disposed on the optical fiberjust outside of grating 522. The clamps 528A and 528B are used tostretch grating 522 by a DC linear motor 530 so as to change the lengthand, therefore, the wavelength at which grating 522 reflects energy. Thegrating 522 would typically be about 10 to 13 millimeters in length. Inorder to stretch the grating 522 the required 1%, the arrangement wouldneed to stretch grating 522 in length 130 micrometers. For the necessaryoptical frequency resolution of one part in 400, the arrangement must beable to control the 130 micrometers to an accuracy of 325 nanometers.This precision is easily within the range of relatively inexpensive DClinear motors. For example, consumer plotters for personal computerapplications have a resolution of 1,000 parts per inch which is morethan an order of magnitude greater resolution than required for thisapplication. Although not shown, various relatively standard mechanicalarrangements could be used for causing the motor 530 to push againstclamps 528A and 528B simultaneously depending upon the setting of themotor 530. Although not shown, the motor 530 would be controlled by thevideo selection computer 50 (refer back momentarily to FIG. 1).

The selected wave length is reflected by grating 522 and is coupled tooptical fiber 204F for converting to the electrical domain by photodiode206 followed by amplifier 208 and the various other components shown inFIG. 4, but not illustrated in FIG. 12.

Turning now to FIG. 13, an arrangement to drop certain channels andinsert other channels is disclosed. In many proposed optical cable TVsystems, it is desirable to re-utilize the optical carriers to aidsystem switching and to add and drop information as required. A methodfor accomplishing this goal is described by D. A. Smith in a paperentitled "Acousto-Optic Filters" presented at LEOS on Nov. 16-19, 1992.However, a superior technique for reuse of optical frequency carriers isshown with reference to FIG. 13.

As shown in FIG. 13, a remove/insert system 540 receives the opticalcarriers or wavelengths on an input optical fiber 540N from thetransmission link and provides a continuation of the transmission linkon output optical fiber 540U. The output fiber 540U will contain thesame channels as the input fiber 540N except that, if desired, one ormore of the optical carriers may be removed by system 540 and reinsertedcarrying different signals thereon. The signals which are added may becarried by the same optical frequency which is dropped and would beprovided from an add channel optical fiber 540A

The optical carriers on input optical fiber 540N pass through isolator542 and into directional coupler 544. Those signals continue on opticalfiber 546 having a series of in-fiber Bragg reflective gratings 548A,548B, through 548J. For ease of discussion, it will be assumed that 10optical carriers or wavelengths are provided and a corresponding 10reflective gratings 548A through 548J would be used. The number ofoptical carriers and corresponding number of reflective gratings couldbe higher or lower. With reference to FIG. 14, the wavelengths W_(A),W_(B), through W_(J) correspond respectively to wavelengths of theoptical carriers coming in on input fiber 540N. Each of the gratings hasan associated resistive heating element controlling or tuning itsfrequency in the same fashion as described in more detail above withrespect to FIG. 9. For ease of illustration, only one of the resistiveheating elements 550A is shown, but numerous such elements would beconnected to a control 552 which would cause the reflectors such as 548Ato either be tuned to a reflection state corresponding to wavelengthW_(A) or to a pass through state where grating 548A would reflectoptical energy only at the A pass through location indicated on FIG. 14.Since there is no optical energy at the wavelength corresponding to theA pass through, all optical energy on optical fiber 546 would passthrough grating 548A when the grating is in its pass through state. Itwill be appreciated that the pass through state is somewhat similar toplacing the gratings in the idler gap state discussed above with respectto FIG. 10. At any rate, each of the gratings is set to either reflect acorresponding wavelength or allow passage of the correspondingwavelength. Assume, for example, that the wavelength W_(B) correspondingto grating 548B is to be dropped from the transmission link, all of thegratings except 548B will be set or tuned to reflect their correspondingwavelengths. Grating 548B will be set to a pass through position suchthat the optical carrier at wavelength W_(B) will appear on the dropchannel end 554. All of the other optical wavelengths will be reflectedback by the corresponding gratings and upon passage through thedirectional coupler 544, will be supplied on output optical fiber 540U.Thus, the optical carrier W_(B) has been dropped.

In addition to dropping the optical carrier, FIG. 13 allows one toreinsert the same optical carrier with different video or data signalsimposed thereon. In particular, carriers containing information which isto be reinserted are applied to add channel optical fiber 540A and passthrough directional coupler 556 on to in-fiber Bragg gratings 558Athrough 558J corresponding respectively to the wavelengths W_(A) throughW_(J) of FIG. 14 and identical in construction with correspondingrespective gratings 548A through 548J. Taking again the example wherewavelength W_(B) has been removed and the same optical wavelength is tobe reinserted carrying different video signals or other information, thecontrol 552 would control the gratings 558A through 558J through variousresistive heating elements (not shown, but same in operation asdescribed in FIG. 9) such that all of the gratings 558A through 558J arein their pass through states except that 558B is tuned to wavelengthW_(B). Therefore, the optical carrier W_(B) received on the add channeloptical fiber 540A is reflected back through directional coupler 556 andpasses through to optical fiber 560 and goes on to output optical fiber540U by way of isolator 562 and coupler 544. (The isolator 562 simplyprevents signals from going from coupler 544 to coupler 556.)Accordingly, the output optical fiber 540U will contain all of theoptical carriers which were allowed to pass through together with anoptical carrier which was dropped and then reinserted with alternatedata, video signals, or other signals imposed thereon.

Although the discussion with respect to FIG. 13 illustrates temperaturetuning of the in-fiber gratings, one could alternately tune thosegratings by mechanical stretching as illustrated and explained withrespect to FIG. 12. Further, it should be noted that the arrangements ofFIGS. 9 and 10 could be modified by tuning the gratings therein bymechanical stretching in similar fashion to FIG. 12. In all cases, thein-fiber gratings may be constructed in known fashion. Such gratings maybe commercially available types produced by United Technologies or themore recently available ATT phase plates.

With reference now to FIG. 15, a technique for local insertion will bedescribed. Distribution of video programs via regional or metropolitanfiber networks makes it increasingly desirable to insert localprogramming or advertising. One method is to electrically take apart thetransmission signal, then reconstruct the signal as is currentlyaccomplished for satellite distribution of video program. Thereconstruction must be done on every channel that requires localinsertions. Channels that do not require local insertions may be passedthrough. The concept of the video switch discussed above primarily withrespect to FIG. 4 together with the other concepts explained previously,show that the present system provides a cable TV system having such alarge number of channels that each channel is a relatively inexpensivecommodity. An advantageous manner of using the video switch networkdiscussed above for insertion will be explained.

Before explaining the specific manner in which the video switch networkof the present invention may be used for advantageous insertion of localprogramming, reference is made to FIG. 15 showing amplifiers 18 of FIG.1, together with more details of the distribution chain. In particular,a Washington, D.C. hub 570 may have metropolitan programming inserted asindicated at 570P. Underneath the hub 570 would be a series of metrocells 572, only two of which are illustrated for simplicity. Under themetro cells 572 are a series of head ends 574. For ease of illustration,only three of the head ends are shown under one of the metro cells 572.As illustrated by 572P, local programming may be inserted into the metrocell. For example, if metro cell 572 corresponded to an area of northernVirginia, local programming might be inserted as indicated at 572P whichwould be of possible interest to residents in the northern Virginiaarea. If the left-most head end 574 of FIG. 15 corresponded to aparticular city in northern Virginia, local programming 574P could beinserted at the head end if the local programming 574P was likely to beof interest only for residents of that particular city.

Turning now to FIG. 16, an arrangement is shown for using the videoswitch network of FIG. 4 in order to accomplish the insertion. Thecomponents in FIG. 16 are numbered in the "600" series with the samelast two digits as the corresponding component, if any, of FIG. 4. Inparticular, a tuneable optical filter 604, diode 606, amplifier 608,multiplier 610 and local oscillator 612 operate as discussed above withrespect to FIG. 4. What is different about FIG. 16 is that a control 630which controls the optical filter 604 and the local oscillator 612,receives channel control data indicating that a substitution of onesignal for another should occur. The optical filter 604 and/or localoscillator 612 are simply switched to an alternate optical carrierand/or radio frequency carrier to the location of the program which isto be inserted. At the time when the viewer is supposed to return to theprimary video signal or channel, the control 630 simply adjusts theoptical filter 604 and/or local oscillator 612 back to the appropriatesettings for the primary source video signal or channel. This may bedone in a manner that is transparent to the viewer. For example, eachprimary video channel or signal could have a low data rate controlchannel associated with it that would tell the video switch what channelwas required for local programming, what time to switch, and how muchtime to allow before returning to the primary source. Alternately, thelocal program could signal the video switch when to return to theprimary program. Local or metropolitan programming could be inserted atthe logical point in the regional tree and branch network as discussedwith respect to FIG. 15. However, local programming could alternately beinserted in the regional network at a higher level than required sincethis may be desirable for operational or logistical reasons and since somany channels are available. For example, with reference to FIG. 15, thelocal programs indicated at 572P and 574P could be inserted with themetro programs 570P at the metropolitan hub 570.

The arrangement of FIG. 16 provides an output from the multiplier 610which would be processed in the same fashion as the output frommultiplier 210 of FIG. 4. Note that the arrangement of FIG. 16 could beused as part of a video switch network 46 of FIG. 4 wherein the videoswitch network was disposed within the head end 26 (refer backmomentarily to FIG. 1). Likewise, that modified video switch networkcould be used in the metro cell 572 of FIG. 15, the metropolitan hub 570of FIG. 15, or more generally the metropolitan hubs 20 of FIG. 1. Inthat case, and with reference to FIG. 1, the regular broadcast channelscould be delivered on optical fibers 26F. If a particular broadcastchannel was going to have a local commercial, the control 630 of FIG. 16would remove the primary channel or source and substitute a differentchannel or source having the commercial thereon.

If desired, the control 630 would not change optical filter to insert anew channel. Instead, the substitute channel would be on the sameoptical carrier as the primary or initial channel. A subcarrier tonewhich carries control information can be placed on the primary channel.This control information will indicate the correct radio frequencysubcarrier channel for tuning in order to obtain the correct localprogramming and will indicate which digital bit stream if 64 QAM isbeing used. In like manner, a control tone on the local programming cantell the video switch when to switch back to the primary programming. Asan alternate, the initial subcarrier control information may indicatethe proper channel for Local programming and a length of time. After acorrect amount of time has elapsed, the signal processing unit switchesback to the primary channel. The method described may be superior to asimilar method discussed herein wherein different optical carriers areused to accomplish the same result. The advantage of this method is thatit avoids possible delays if the optical tuner has a slow response timein tuning from one optical frequency to another.

With reference now to FIG. 17, a further arrangement is shown forinsertion of local programming. This arrangement, which could, forexample, be located in the metro hub 570 of FIG. 15, receives inputsfrom optical fiber 18F (refer also back to FIGS. 15 and FIG. 1 withrespect to the location of this optical fiber). The optical fiber 18Fsupplies signals to a plurality of insertion networks 740, only two ofwhich are illustrated for simplicity. Since each of the insertionnetworks 740 includes numerous components identical to the components ofthe selector 202 of FIG. 4, components in the arrangement of FIG. 17 arelabeled in the "700" series with the same last two digits as thecorresponding component, if any, from FIG. 4. Thus, a tuneable opticalfilter 704 operates in similar fashion to optical filter 204 of FIG. 4.For ease of illustration, the control for the optical filter is notillustrated, but control may be accomplished as discussed previouslywith respect to FIG. 4. Photodiode 706 and amplifier 708 operate in thesame fashion as the corresponding components of FIG. 4. The output fromamplifier 708 is supplied to a low pass filter 742 which leaves theradio frequency carriers carrying the regional signals in place.Referring momentarily to FIG. 18, the operation of filter 742 will bebetter understood.

In the system as discussed, subcarrier multiplexing is used inconjunction with a modulation format such as 64 QAM on six MHz channelspacing. If two GHz of radio frequency bandwidth per optical carrier isused, about three hundred of the six MHz channels are available.Assuming that 10 video channels are used for each radio frequencysubcarrier, three thousand video channels can be used on a singleoptical carrier. With such large channel capacity, a certain number ofthe primary video programs which originate at the regional head end willrequire the insertion of local programming. In this case, space may beleft in the radio frequency domain for additional channels which can beadded at the metro level. As shown by the radio frequency spectrum ofFIG. 18, the regional radio frequency carriers will pass through thefilter 742, whereas frequencies above the regional carriers will beblocked by filter 742. The local programming may be added at thosefrequencies in the manner illustrated in FIG. 17.

The regional carriers passing through 742 are supplied to an adder 744which also receives local programming on radio frequency carriers abovethe cutoff frequency of the filter 742. For ease of illustration, FIG.17 does not illustrate the actual programming signals being supplied toa modulator (QAM, or other desired type), but this may be accomplishedin similar fashion to that discussed with respect to FIG. 2. Althoughnot shown, one could additionally have the local programming timedivision multiplexed in the fashion illustrated in FIG. 2. The adder 744combines the regional carriers which are passing through insertionnetwork 740 with the additional carriers having the local programmingthereon and passes these signals through to a modulator 746 modulatinglaser energy from laser 748. The output of modulator 746 is therefore anoptical carrier having the radio frequency subcarriers corresponding toboth the regional radio frequency carriers and the local programmingcarriers of FIG. 18. The output of modulator 746 is supplied by opticalfiber to coupler 750, which is an optical signal combiner combining thevarious optical carriers from the different insertion networks 740 andsupplying them on an output optical fiber 750F which may be used todistribute programming from the regional head end. It should beappreciated that the arrangement of FIG. 17 avoids the need for totallydisassembling the numerous channels. Although the channels are convertedfrom the optical domain to the electrical domain, the regional radiofrequency carriers are allowed to pass through without requiring furtherdisassembly.

With reference now to FIG. 19, a further possible modification of thevideo switch network of FIG. 4 is illustrated. The arrangement of FIG.19 has numbers in the "800" series with the same last two digits as thecorresponding component, if any, from the arrangement of FIG. 4. Thearrangement of FIG. 19 continues the strategy of the present system ofproviding resource sharing such that the more expensive hardware whichwould be used only a fraction of the time by any one subscriber can belocated at the video switch network. In FIG. 19, tuneable filter 804,photodiode 806, multiplier 810, and local oscillator 812 operate asbefore. A demodulator 840 demodulates the output of multiplier 810 and,in turn, supplies its output to a computer 842 connected by a bus 844 toRAM 846, disc drive 848, computer 850 receiving control information froma subscriber, microprocessor 852, and computer 854. The computer 854also provides an output to a 64 QAM modulator 856 connected tomultiplier 818 receiving an input from local oscillator 820. Themultiplier 818 supplies its output to an adder 822 which receivessignals from other such multipliers 818, only one other one being shownfor ease of illustration. The components between optical filter 804 andmultiplier 818 including local oscillator 820 may be substituted foreach of the selector blocks 202 in the arrangement of FIG. 4.

The arrangement of FIG. 19 allows the subscriber to access acomputerized arrangement to improve the functions of the video switchnetwork. For example, if the subscriber is watching a movie, thecomputer arrangement can provide a forward, reverse, and freeze framecapability as with a video cassette recorder. By having sufficientmemory within the video switch, the subscriber can obtain thosefeatures. Further, the computerized video switch network of FIG. 19 willfacilitate multi-media games or other required computer functions.

Turning now to FIG. 20, more details of the on demand program center 34of FIG. 1 will be discussed. Initially, it should be emphasized that theon demand program center 34 may be located at the same location as theregional transmitter 12. Therefore, a single such program center 34 maybe used for all of the metropolitan areas within a particular regionalzone. This furthers the present system's use of shared resources wherebythe most expensive resources are shared over the widest possible area.It will also be appreciated that the tremendous channel capacity of thepresent system such that each channel is relatively inexpensive, allowsone to set up the program center 34 according to the present invention.

The program center 34 will provide movies or other pay per view or videoon demand programs. In the discussion which follows, the emphasis willbe on movies, but other programs could of course use the concepts whichwill be discussed. A series of hard drives 900A and 900B are controlledby a computer processor 902. Each of the hard drives 900A has storedthereon a complete copy of a 100 minute movie. In similar fashion, eachof the hard drives 900B have digital versions of a movie, different fromthe movies stored on hard drives 900A. With reference to the top harddrive 900A, 10 outputs 904 symbolically indicate that the hard drive900A is being accessed sufficiently fast to provide time-offset versionsof the same movie corresponding to 10 different times. In other words,accessing the hard drive sufficiently fast may provide 10 signalscorresponding to time-shifted versions of the movie stored on hard drive900A. If desired, the 10 outputs 904, only three of which areillustrated, may be obtained by supplying the actual output of the harddrive to a time division demultiplexor which switches the actual outputof the hard drive onto 10 different sample and hold circuits whoseoutputs are the actual lines 904. However, various known techniques canalternately be used for simply supplying the 10 different time-shiftedversions of the movie from hard drive 900A.

With reference now to FIG. 21, the time shifting is illustrated byassuming that each of the outputs 904 corresponds to one channel.Channel one has the movie starting at a particular point, channel twohas the movie starting at a point occurring later in time than the startof channel one, and so forth. If the ten outputs from the top hard drive900A have staggered starts every five minutes and the 10 outputs fromthe other hard drive 900A are staggered every five minutes starting 55minutes after the program start of the first output 904 of top harddrive 900A, the 20 output lines from the pair of hard drives 900Aprovide 20 different time-shifted signals corresponding to the movie.For a 100 minute movie, 20 time-shifted signals allows the system to bestarting the movie within five minutes of any subscriber's request aswill be described in more detail below. As soon as the movie has beencompleted on a particular one of the output lines from hard drives 900A,the movie is restarted. Thus, the movie is being continuously played onthe 20 output lines from the pair of hard drives 900A. In similarfashion, a different movie stored on hard drives 900B may providetime-shifted signals corresponding to that movie every five minutes.

The 20 time-shifted signals corresponding to each of the movies storedon the various hard drives (more than the two pairs illustrated) areprovided to a laser network 906 of which laser 42 (refer backmomentarily to FIG. 1) is a part. The laser network 906 provides timedivision multiplexing, quadrature amplitude modulation (or other radiofrequency modulation) and optical wavelength modulation using the sametechnique described in detail above with respect to the first lasernetwork 52 of FIG. 2. Considering that the technique allows a singlelaser to carry 3,000 different video channels or signals, a single suchlaser network 906 could provide 150 different 100 minute moviesstaggered at five minute starting times. Thus, 150 pairs of hard driveswould be connected to the laser network 906. Considering FIG. 20 inconjunction with FIG. 2, it may be noted that the 10 different outputlines symbolically illustrated from hard drive 900A might be dispensedwith and the single output line from such a hard drive could be supplieddirectly into the quadrature amplitude modulator such as 56 of FIG. 2.In other words, the hard drive 900A would be inherently performing thetime division multiplexing and would not require a multiplexor such as54 of FIG. 2.

A second laser network 906 is shown below the top laser network 906. Forease of illustration, the various hard drives and computer processorassociated with that laser network are not shown, but it will beunderstood that this second laser network 906 would operate in the samefashion as discussed. Further, numerous additional laser networks couldbe used. If 100 such laser networks were used, the program center 34could provide continuous versions of 15,000 movies, any particular moviestarting no more than five minutes away from a particular time. Themovies would be supplied by optical fibers 44F to the video switchnetworks 46 shown in FIG. 1 and discussed in more detail above.

An important feature of the present system is that the hard drives, forexample, 30,000 corresponding to 15,000 pairs for 15,000 movies, can belocated at the regional hub. Therefore, the relatively large expense ofproviding the massive storage corresponding to all of these movies maybe spread over the numerous subscribers on the various metropolitan hubs20 (refer back to FIG. 1). It should further be appreciated that FIG. 20does show the use of hard drives, but other storage devices might beused to implement the concept of the present invention whereby moviesare run continuously and the video switch network 46 of FIG. 1 is usedto switch the movies to a particular node upon request by a subscriber.

Turning now to FIG. 22, a method will be described to minimize dead airtime after a subscriber has requested one of the pay per view or ondemand movies stored with the program center 34. FIG. 22 shows a timeline illustrating channels one and two and having a subscriber lineindicating that the subscriber requests a particular movie after theprogram has started on channel one and before the start of the programon channel two. After an initial delay, a preview may be supplied to thesubscriber. The preview would be delivered by control of the videoswitch network 46 discussed above with respect to FIG. 1. The previewmay be one of a large series of previews stored upon hard drives such asthose discussed with respect to FIG. 20. The previews may be various 30second or one minute long previews stored upon hard drives and deliveredby the same arrangement discussed with respect to FIG. 20. Since thepreviews are supplied in time-shifted or staggered versions in the samefashion as the movies or other programs which are requested, the initialdelay before the preview starts may be very slight. As indicated on thesubscriber line of FIG. 22, the video switch network 46 of FIG. 1 willswitch to channel two upon completion of the preview time and when theprogram start on channel two actually occurs.

With reference now to FIG. 23, a slight modification of the arrangementof FIG. 20 is illustrated in order to hold down the cost of the harddrives. Since the hard drives used in FIG. 20 have a cost which isdependent upon the storage capacity, the arrangement of FIG. 23 mayprovide the same five minute staggered starts discussed with respect toFIG. 20, while reducing the storage capacity and, thus, the cost. Thehard drive 910 stores a complete version of the movie in question andhas 10 outputs 912 on which the 10 time-staggered versions are provided.However, the hard drive 914 has only half the storage capacity of harddrive 910. Basically, the hard drive 914, which can only hold one halfof the movie, is used as a delay line. Hard drive 914 receives an inputon line 916 which comes from the last-to-start output line 912 of harddrive 910. Accordingly, when the 10 time-staggered output lines 918 ofhard drive 914 are in the first half of the movie, the output lines 912of hard drive 910 will be in the second half of the movie. At any giventime, different portions of the movie will be provided by the differenthard drives 910 and 914. Since hard drive 914 has only half the storagecapacity of hard drive 910, it may be significantly less expensive thanhard drive 910.

It should be appreciated that the arrangement of FIG. 23 could berepeated for more than two hard drives. Although not shown, an outputfrom hard drive 914 could be fed as an input to a third hard drivehaving one half the capacity of hard drive 914. This will allow a largernumber of time-staggered versions of a particular program, such as avery popular movie, to be provided. It will of course be readilyappreciated that the five minute time for staggering the starts could bemore or less depending upon various factors.

Turning now to FIG. 24, a modification of the video switch network 936of FIG. 4 is illustrated for providing video by way of twisted pairtelephone wires 920A and 920B. A tuneable optical filter 922, photodiode924, amplifier 926, multiplier 928, and local oscillator 930 may operatein the same fashion as described with respect to FIG. 4. The output frommultiplier 928 is supplied to a time division demultiplexor 932 whichselects one video signal for application to a telephone-compatiblemodulator 934. The modulator 934 uses a carrierless type of modulationas developed by one or more telephone companies in order to provide avideo channel upon twisted pair wires such as 920A and 920B. Inparticular, the telephone system includes an extra twisted pair and thetelephone modulator 934 is a previously developed device to deliver avideo signal over that twisted pair. The output of modulator 934 isswitched by switch network 936 to one or more of the twisted pairs 920A,920B, or any of numerous additional twisted pairs, not illustrated.Although not illustrated, the components between filter 922 andtelephone modulator 934 could be duplicated with, for example, 10 suchnetworks arranged to provide 10 telephone-format modulated signals at 10different inputs to the switch network 936. The switch network 936 wouldthen simply be set up to supply any of, for example, 100 twisted pairsconnected to outputs of the switch network with any of 10 differentinputs supplied to the switch network 936.

Turning now to FIG. 25, an optical amplifier arrangement is shown. Apump laser 940 receives input from optical fiber 940N. The outputoptical fiber 940F, including the laser energy from the pump laser andthe optical signals received at the amplifier input 940N are supplied toan Erbium fiber 942 connected to a first port 944F of a circulator 944.The port 944F also supplies laser energy from pump laser 946 by way of aport 944R of circulator 944. The laser energy supplied by the pumplasers 940 and 946 excite the Erbium ions such that the optical signalsreceived on input fiber 940N will be provided in amplified form out ofport 944S of circulator 944. However, the gain of this optical amplifiermay be nonuniform over wavelength as indicated by the gain curve of FIG.26. In-fiber Bragg gratings 948, 948A, and optical attenuators 950Bthrough 950J are used to filter out the laser energy inserted by thepump lasers 940 and 946 and are used to provide a relatively flatadjusted gain curve as shown in FIG. 26. In particular, the signalscoming in on amplifier input 940N include ten different opticalwavelengths as previously discussed. Therefore, 10 of the gratings 948Athrough 948J would be included, one for each of the wavelengths. Thegratings would be used to reflect the optical energy at thecorresponding wavelength. The gratings 948A through 948J could betuneable in the same fashion as the gratings discussed above, althoughthe gratings would preferably simply be maintained at a sufficientlystable temperature that they would maintain their reflectivecharacteristics for the corresponding wavelength.

As shown on the small portion of the gain curve illustrated in FIG. 26,the wavelength corresponding to point A has the lowest gain from theoptical amplifier. Therefore, optical energy at that wavelength isreflected back to port 944S of circulator 944 without any attenuation.Since the wavelength corresponding to point B of FIG. 26, which alsocorresponds to grating 948B, has a higher gain than the wavelengthcorresponding to point A, an adjustable attenuator 950B attentuates theoptical energy at the wavelength corresponding to point B. Consideringthat that optical energy passes through attenuator 950B twice, bringingthe gain down from point B to point B' in FIG. 26 requires thatattenuator 950B provide one half of the attenuation required to movefrom point B to point B' in FIG. 26.

Optical energy at the wavelength corresponding to point C of FIG. 26 maybe adjusted down to the flat response curve or adjusted gain of FIG. 26at point C' by having attenuator 950C provide an attenuation equal toone half of the difference in gain between point B and point C. By usingthe same technique at the different wavelengths, one can provide anadjusted gain curve which is relatively flat. The various reflectedoptical wavelengths or carriers enter the circulator at 944, whereas thelaser energy from pump lasers 940 and 946 would be at differentwavelengths and not reflected back to that port. The optical energyentering at 944S is supplied by circulator 944 to a port 944T andprovides the output at optical fiber 952. Optionally, a coupler 954supplies the output to an optical spectrum analyzer 956 which isconnected to a microprocessor 958. The microprocessor in turn controlsthe adjustable optical attenuators 950B through 950J so as to provide aflat response curve corresponding to the adjusted gain illustrated inFIG. 26.

For the above arrangement, it is assumed that all of the gratings havethe same reflectivity. The control of the optical attenuators can beaccomplished using known techniques. In the specific aspect of thepresent invention, the optical attenuator 950B would first be adjusteduntil the gain at point B was the same as the gain at point A. This isdone by making the attenuator 950B correspond to one half of thedifference in gain between points A and B. The attenuator 950C is thenadjusted until the gain at that wavelength corresponds to the gain atpoint A. This process may be repeated for all 10 (or some other number)of the optical carriers in such a wave division multiplexed opticalsystem.

By providing the spectrum analyzer 956 and microprocessor 958, the gainof the amplifier (all those components numbered between 940N and 954 canbe maintained to provide a flat response curve.

The amplifier of FIG. 25 is an improvement of one disclosed and claimedin the present inventor's prior application U.S. Ser. No. 07/919,823filed Jul. 27, 1992 now U.S. Pat. No. 5,283,686, entitled "OpticalSystem With Grating Reflector" and hereby incorporated by reference. Theamplifier of FIG. 25 may be used as optical amplifiers 18 and 62 of FIG.2 and at other places in the system.

The optical source at the head end of the system can be configured usingeither of the two techniques shown in FIG. 27. The techniquecorresponding to 960A is use of a laser at the head end as alsopreviously shown and discussed herein. However, one could alternatelyuse a tuneable laser diode residing at the metrocell as shown for 960B.The advantage of using a tuneable laser diode is that the tuner is ashared resource. In initial deployment, not all of the nodes willrequire simultaneous operation (because of lack of traffic). Therefore,a tuneable laser can address the grating at the head end which isrequesting data or video transmission. As service demand grows,additional tuneable lasers can be added or a dedicated laser can beadded at the head end. In order to efficiently multiplex or switch therequired information, it is desirable to carry as many radio frequencyfrequency domain multiplexed subcarrier channels as possible on a singleoptical wavelength. The radio frequency modulation optical carrier. Inthis case, a number of nodes could be daisy chained together as shown inFIG. 28 (a variation on 960B of FIG. 27). The daisy chain arrangementhas optical fibers similar to 128F of FIG. 3 linked together betweenmodulators 962 corresponding to different ones of the nodes.

In the daisy chain case, it is required that none of the radio frequencysubcarriers used at node i of FIG. 28 also be used at node i plus 1 orat node i plus 2. As data and video traffic increase, each node can bereconfigured to have a unique optical carrier. If each optical carriersupports 2 GHz of radio frequency bandwidth and analog FM placed onradio frequency subcarriers spaced 40 MHz apart, there would be 50 radiofrequency subcarrier channels. If, in addition, each metro cell has 100optical carriers, the metro cell is capable of carrying 5,000simultaneous calls. Assuming an initial network utilization of 1%, thenetwork is capable of serving 500,000 subscribers. If each regional cellhas 100 metro cells, each region could service 50 million subscribers.Even at 10% network utilization, a regional cell could service fivemillion subscribers. If network service demands exceed that described,the capacity can be expanded by increasing the number of opticalcarriers and/or utilizing space division multiplexing.

In the case of this type of video phone information that originates atthe node and which is stay within its own metro cell, analog FMmodulation format is sufficient. Video information going to other metrocell or out of the region can be digitally compressed and concentrated.

FIG. 29 shows a metro concentrator which may provide the concentrationand compression as discussed. A regional concentrator could be made inessentially the same fashion.

With reference to FIG. 30, if network demand is sufficient or if it isnecessary to transport digital data to another region, base band digitaltransmission can be utilized as shown. An alternate arrangement for suchtransmission is shown in FIG. 31.

With reference now to FIG. 32, the information may be placed on thefibers at the node using the illustrated technique. If the informationis digital data, it may be placed on the fiber node as shown in FIG. 33.

With reference to FIG. 34, video phone information and digital dataoriginating at the home of the subscribers (upstream data) may beconcentrated into base band digital data for transmission on theregional bus (if service demands are great, high composite data rate).Data sent on the national network will most likely require base banddigital because of the large distances required. Once the data reachesthe required region, the data can be demultiplexed into data rates whichmore nearly match the rates required by the subscriber.

With reference now to FIG. 35, data generated within the metro cells fortransmission for the national network (possibly the regional network ifdemand is sufficient) may be processed as shown in FIG. 25. It should benoted that the tuneable optical filter for selection of time divisionmultiplex data and the tuneable output frequency laser allow dynamicnetwork allocation of resources. The output frequency of laser canselect data destination as shown in the example of FIG. 35. The outputsat the regions at the far right of FIG. 35 may provide either reuse ofoptical carrier frequencies by space division multiplexing or more thanone wavelength for transmission to each different region.

Various specific constructions and details have been given in thepresent specification, but it is to be understood that these are forillustrative purposes only. Various modifications and adaptations of thepresent invention will be apparent to those of skill in the art.Accordingly, the present invention should be determined by reference tothe claims appended hereto.

What is claimed is:
 1. A method for removing and inserting opticalcarriers in an optical transmission path comprising:providing an opticaltransmission path; transmitting a plurality of optical carriers alongthe optical transmission path; providing first and second directionaloptical transfer devices optically coupled to the optical transmissionpath, each of the first and second optical transfer devices having atleast three optical ports for the input and output of optical signals;optically coupling at least one Bragg grating to an output port of atleast one of the first and second optical transfer devices, the at leastone Bragg grating reflecting an optical carrier wavelength; inputtingthe plurality of optical carriers to the first optical transfer device;removing an optical carrier from the plurality of optical carriers inputto the first optical transfer device and outputting the removed opticalcarrier through a first transfer device optical port; outputting theremaining optical channels through an optical port of the first opticaltransfer device, the removed optical channel and the remaining opticalchannels being output through different optical ports of the firstoptical transfer device; inputting an insertion optical carrier throughan optical port of the second optical transfer device, the insertionoptical carrier passing through the second optical transfer device portcoupled to the Bragg grating; reflecting the insertion optical carrierwith the Bragg grating back through the second transfer device opticalport coupled to the Bragg grating; outputting the reflected insertionoptical carrier from the second optical transfer device such that theinsertion optical carrier is added to the optical carriers output by thefirst optical transfer device.
 2. A method for removing and insertingoptical carriers in an optical transmission path according to claim 1wherein the removed optical carrier and the insertion optical carrierhave the same wavelength.